1. Field
This application is in the technical field of wireless communications, more particularly to a system or method for increasing capacity or usability of a wireless channel.
2. Prior Art
Wireless signal transmission utilizing electromagnetic radiation inherently requires dealing with the effects of electromagnetic (EM) field polarization of the transmitted signal. As radio frequency (RF) signals are manifested as transverse waves, the polarization of the electric and magnetic fields is determined by the phasing of the electric and magnetic field components of the wave. Typical transmitter/receiver pairs utilize horizontal, vertical, circular, or sometimes elliptical polarization of RF signals. Typically, selection of an appropriate polarization in a design involves tradeoffs between simplicity of implementation and the signal-to-noise ratios (SNRs) of the transmitter/receiver pair. Increased SNR can allow for increased data rates within the allocated RF spectrum to an extent, but this is limited ultimately by the total allocated bandwidth.
Several methods for using polarization to increase RF channel data capacity currently exist. These methods typically employ matching of polarization between transmitter and receiver, as well as creating multiple channels by transmitting on overlapping frequency bands at different polarizations. Matched polarization typically increases the gain and SNR of a channel. If both the receiver and transmitter antenna orientations are known, transmitted energy can be focused into a specific polarization that matches that of the receiver, typically allowing greater gain or link margin. Prior work in this area is shown by Shapira et al. in U.S. Pat. No. 6,889,061 and Dent in U.S. Pat. No. 5,724,666 which entails the use of matched polarization to increase SNR. In both cases, identical signals are transmitted on multiple polarizations to increase signal strength. However, this does not inherently lead to increased data rate or more efficient use of frequency allocations and cannot increase the absolute maximum capacity of the channel.
Adaptive polarization techniques, described in U.S. Pat. Nos. 6,646,615, 6,411,824, 6,963,619, and 6,889,061 to Andrews et al., Eidson, Gesbert et al., and Shapira et al., respectively, show the use of various techniques to detect the polarization of the incoming signal. This is used to increase the SNR of the link at a base station and in some cases used to boost the SNR of the link to the mobile station by transmitting on the same polarization. Gesbert et al. use a method of singular value decomposition for the detection of the polarization. Andrews et al. take advantage of scattering of incoming signals. All of these techniques can be used to increase signal reception and increase in spatial diversity. However, none of these techniques are able to increase the maximum theoretical data rate of a single link beyond the frequency allocation nor are they able to take advantage of any knowledge the mobile transceiver may have of its own orientation to the tower.
Transmitting data at multiple polarizations on the same spectrum allows for an increase in data rate up to a theoretical maximum of double the normal bandwidth. This is typically used in applications such as satellite television where RF spectrum is valuable and both receiver and transmitter orientations are well known. This is shown in U.S. Pat. No. 5,760,740 to Blodgett where decoupled polarization channels are isolated to increase data capacity, typically for satellite applications. This is also shown in U.S. Pat. No. 6,445,926 to Boch et al. which shows how this technique can be used for other types of fixed receivers. However, both of these implementations require fixed knowledge about the orientation of both transceivers.
Using multiple polarizations to increase SNR and using fixed antenna orientations to increase data rate are both limited by the fact that the relative orientation of the transmit and receive antennas must be known. Therefore, these methods are generally only applicable to stationary receivers. Smaller devices, such as mobile phones and small satellite receivers therefore cannot take advantage of these polarization-based techniques for increased link quality and data rate.
Methods exist for increasing the apparent channel capacity using polarization techniques for mobile devices. This can be seen in U.S. Pat. No. 6,418,316 to Hildebrand et al. which uses geographical separation of different polarizations to decrease interference and allow additional channel capacity for the main transceiver. A similar technique is shown in a patent application by McKay, S R (Publication No. US2007/0010198). However, this method only applies to fixed transceivers with a large number of users (such as cellular phone towers) and only adds useful capacity by splitting the transmit field into separate physical areas based on polarization. This reduces interference on the channel and therefore indirectly increases the link quality for the large transceiver, but does not directly provide for increased data rate to the mobile receiver beyond the spectral bandwidth limit. Although this provides increased channel availability when multiple users are involved, the maximum data rate for any single mobile use remains the same.